The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
Scuba diving is a form of underwater diving in which a diver uses a self-contained underwater breathing apparatus to breathe underwater. Buddy and team diving procedures are intended to ensure that a scuba diver who gets into difficulty underwater is in the presence of a similarly equipped person who can offer assistance. Divers may communicate basic and emergency information using hand signals, light signals, and rope signals, and more complex messages may be written on waterproof slates. Satellite-based positioning systems, such as the global positioning system GPS, enable determining an accurate location on the Earth's surface. However, GPS signals do not propagate underwater.
Sonar refers to a technique using sound propagation to navigate, communicate with or detect objects on or under the surface of water. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). Ultrasound is a sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.